This invention relates to an image forming apparatus for processing image data and forming a tone image.
With an apparatus of this kind, a screen half-toned (converted into half-tone dot) image is formed by a dither method, light-emission time control method, brightness modulation control method, etc. With methods such as the dither method, however, the tonality and resolution of the image decline and there is no improvement in image quality. With the light-emission time control method and brightness modulation method, screen half-toning is insufficient and impediments are encountered at print-out.
When an original image for printing presses is formed or an ordinary copy is made using the conventional apparatus of the above-described type, dot formation is carried out without changing the method of image representation (i.e., the method of image reproduction). When the original image printing presses is produced, therefore, printing dot size is not constant and the original image cannot be used as such an original image for printing presses.
Further, in the conventional apparatus of the foregoing type, an enhancement in image quality is attempted by applying so-called screen angle control in a printing technique. With the dither method, however, tonality and resolution decline. Though tonality and resolution are improved with a dot modulation method or the brightness modulation method, performing image processing for each and every dot is fundamental and, in consequence, the screen angle is greatly limited by the main scanning and subsidiary scanning methods and a screen angle cannot be applied at will.
With the conventional apparatus of the foregoing type, an irregularity shading occurs when there is a deviation in speed combination between a photosensitive body drive mechanism and a laser irradiation mechanism, by way of example. When an image is outputted, therefore, the screen half-tones and irregular shade interfere and the image deteriorates.
With the abovementioned apparatus, tonal representation of an image is performed by the dither method, light-emission time control method, brightness modulation method, etc. However, tone jump occurs with these methods owing to an electrophotographic characteristic at the time of image formation. In addition, color tone changes unless the origin of the screen half-tones are aligned when the screen half-tones are formed.
With the abovementioned apparatus, color print images for toners of the three colors yellow (Y), magenta (M) and cyan (C) along with the color black (Bk) for inking are color-synthesized in the sequence C.fwdarw.M.fwdarw.Y.fwdarw.Bk or M.fwdarw.C.fwdarw.Y.fwdarw.Bk. However, when colors are synthesized in these sequences, the Y toner, which has high reflectance and relative luminous efficiency, rises to the top layer (see FIG. 15). As a result, the image tends to have a strong yellowish color and color reproducibiilty declines.
In order to obtain a reproduced image having a high quality, the applicant has already proposed the cases indicated below, however, further improvements are sought: U.S. Pat. Nos. 4,763,199, 4,800,442, 4,811,037, 4,819,006, 4,847,654, 4,847,695, 4,870,499, 4,873,537, 4,897,734, 4,980,757 and 4,987,495, and U.S. patent applications Ser. Nos. 07/378,488 (July 12, 1989) and 07/090,238 (Aug. 25, 1987).